Post-wildfire Analysis of Avalanche Hazard

August 7, 2024 By Cam Campbell, Brian Gould and Scott Thumlert

Introduction

In recent years, western Canada has experienced the most severe wildfire seasons in recorded history. Wildfires have impacted avalanche paths that have been previously assessed for avalanche hazard. Considering the drastic alteration of the landscape that occurs from wildfire, it is reasonable to consider avalanche characteristics will also be altered, creating uncertainty in previous avalanche hazard assessments.

As is well understood, forest cover is considered a key terrain feature influencing where and how avalanches initiate and flow. Forest cover reduces the probability of avalanche formation, with tree density as little as 200 stems/ha (i.e., 7.0 m average spacing) sufficient to inhibit destructive avalanches from starting (Schweizer et al., 2003). Once started, dense forest cover and can slow or even stop small– to medium–sized avalanches (Teich et al., 2012). After a wildfire, these effects can be altered for several decades or longer if forest regeneration is subsequently inhibited by an increase in avalanche frequency and size. Therefore, previously assessed avalanche paths that have been affected by wildfires may require reassessment to determine if deforestation alters avalanche hazard.

This article reviews the effects of forest cover on avalanche hazard and introduces methods to determine any increase in potential avalanche magnitude for paths burned by wildfire.

Avalanche Path Wildfire

Figure 1: Avalanche path in Waterton Lakes National Park which was burned by the 2017 Kenow wildfire.

Effects of Forest Cover on Avalanche Hazard

In general, forest cover reduces both the frequency and magnitude of avalanches through three main effects: 1) modifying snowpack structure, 2) providing structural support to the snowpack, and 3) modifying the dynamics of flowing avalanches.

Modifying Snowpack Structure

Forest canopies create a microclimate that alters height, stratigraphy, and spatial variation of the snowpack, and reduces the likelihood of avalanche initiation and propagation through:

moderation of the radiation balance at the snow surface, which leads to fewer persistent weak layers;

interception and subsequent release of snowfall (i.e., ‘tree-bombing’) that can interrupt the homogeneity of the snowpack and limit the extent of continuous weak layers; and

reduction of wind-transported snow, which limits the formation of deep and cohesive slabs.

In addition, trees that are either buried or extend through the snowpack can disrupt the homogeneity and limit the continuity of weak layers.

Providing Structural Support

Tree stems can effectively anchor the snowpack and reduce the likelihood of slab release. This structural support depends strongly on the tree species (Bebi et al., 2009) as well as forest density. Rudolf-Miklau et al., (2011) provide a good guideline of 500 stems/ha (i.e., 4.5 m average spacing) for 30º slopes and 1000 stems/ha (i.e., 3.2 m average spacing) for 40º slopes, which is supported by Weir (2002).

Modifying Flowing Avalanches

A dense forest can slow or stop small– to medium–sized avalanches through detrainment and increased friction (Teich et al., 2012); however, this effect is reduced for large avalanches. Bartlet and Stockli (2001) use a conservation of energy approach to explain why large avalanches can destroy forests without significant deceleration, and Margreth (2004) states that broken trees can add mass to very large avalanches and actually increase the runout distance.

Methods to Assess the Effect of Wildfire on Avalanche Hazard

Post-wildfire reassessment of avalanche hazard attempts to answer the following questions:

What were the characteristics and extent of the pre-burn forest cover? Was the forest cover located in the starting zone, track, and/or runout zone? What was the tree species, crown diameter, and spacing? Spatial vegetation data (e.g., BC’s Vegetation Resource Inventory) can be a valuable resource.

What was the severity and extent of the wildfire? What was the spatial extent of the burnt forest? How severely did the canopy and the root systems of the forest get burned? How did the ground roughness change?

How are the avalanche characteristics likely to change? Will avalanches become more frequent? Will avalanches become more destructive?

The following two sections introduce methods that could be used to analyze any change in avalanche magnitude as a result of deforestation from wildfire.

Air Photo and Satellite Imagery Interpretation

Historical air photos and satellite imagery, specifically for locations where avalanches interact with forests creating trim lines, provide a rich dataset of large avalanches and are often used in avalanche hazard assessment (Jamieson et al., 2018). Information about historical wildfires including areal extents, severity of burn and specific dates are often available for significant wildfires. The historical imagery is especially valuable if it is representative of the avalanche paths being reassessed, and shows pre– and post-wildfire trim lines.

As an example, while revising avalanche path maps for Highway 93 South, we found an excellent series of historical air photos with approximately one high quality photo every decade from the 1940s to early 2000s. These air photos show obvious trim lines from avalanches on the southeast face of Mt. Whymper before and after a large wildfire that occurred during the summer of 1968. The trim lines have increased in runout distance when comparing the 1978 post-wildfire to the 1966 pre-wildfire photos (Figure 2).

Weather records show that the early 1970s were large snowfall years, so it would be possible to explain the increased runout distance with larger snow years producing larger avalanches. However, when examining the avalanche paths off the southwest face of Boom Mountain, immediately to the northeast of Mt Whymper, where the forest was unaffected by wildfire, no discernible increase in runout distance is observed.

Avalanche Paths

Figure 2: 1966 pre-wildfire air photo (left) and 1978 post-wildfire air photo (right) of the Mt. Whymper avalanche paths (bottom left of the photographs) and avalanche paths off Boom Mountain (centre and top of the photographs). Pre-wildfire runout extents are outlined in green and post-wildfire runout extents are in red.

RAMMS Simulations

The two-dimensional RAMMS – Rapid Mass Movement Simulation physical-dynamic model (Christen et al., 2010) can provide insight into effect of wildfire on impact pressure and runout extent by simulating pre- and post-wildfire scenarios. First, the simulation is fit to the pre-wildfire assessment by adjusting release depth and friction parameters, while defining release areas to account for the structural support and snowpack modification provided by forest cover in the starting zone, and increasing the friction where avalanches flow into densely forested areas and have impact pressures of < 100 kPa (i.e., small- to medium-sized avalanches). Then with all other parameters remaining constant, the post-wildfire scenario is simulated by increasing the release area to include the deforested areas within the starting zone, and the areas of increased friction in the track and runout zone are removed where they are affected by wildfire.

As an example, we used RAMMS to reassess avalanche paths that threaten the town of Waterton in southwestern Alberta. RAMMS model simulations were initially fit to previous avalanche hazard zones by adjusting release volume while incorporating increased friction in areas of the path that were previously forested. The post-wildfire scenario was then simulated by using the same input parameters without the increased friction for forested areas, and the changes to impact pressure and runout distances were analyzed (Figure 3).

Avalanche Path Wildefire

Figure 3: RAMMS simulation results shown in yellow-orange shading with (left) and without (right) forest cover which is shown by green shading. The pre-wildfire avalanche hazard zones (dashed lines) are also shown, as well as modeled 30 kPa impact pressure threshold (black pixels in RAMMS model results).

Discussion

We present an overview how large wildfires alter avalanche terrain and introduce some ideas for assessing any resulting changes to potential avalanche magnitude. A complete reassessment of avalanche hazard would also consider any resulting changes to avalanche frequency, which would include a detailed analysis of snow climate and avalanche winter regime and well as an assessment of wind exposure.

Given the increased risk of wildfire due to climate change (IPCC, 2018) and the increasing fuel loads from past wildfire suppression (e.g., Perry et al., 2011; Hessburg et al., 2016), the frequency and severity of wildfire altering avalanche terrain is expected to increase. This potentially drastic alteration of a key terrain parameter can create uncertainty in previous avalanche hazard assessments. Therefore, post-wildfire reassessments of avalanche hazard will likely become more common. The methods presented here, in combination with other methods, may be useful to determine any increase in avalanche hazard associated with deforestation from wildfire.

Post Wildfire Avalanche Zoning Project

Waterton Townsite Pre/Post Wildfire Assessment & Zoning

Parks Canada – Waterton Lakes National Park

Waterton, AB, Canada

Acknowledgements and References

The authors would like to acknowledge Parks Canada for approving the use of Mt Whymper and Waterton as examples.

References

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Post-wildfire Analysis of Avalanche Hazard

Introduction